Some True Facts
Most people think that having a pouch is the main characteristic of a marsupial. This is not strictly true.
According to biologists, marsupials are unique because of their reproductive structures and system.
Female marsupials have one opening through which both urine is passed and babies are born. In the middle of the space behind the opening is the end of the urine tube, or urethra. Marsupials have two vaginas or openings to the tubes where the eggs will be fertilized and develop into babies. (There are slight variations between different marsupial species. Some have a little pocket shaped structure in between the vaginas, which is often referred to as a 'third vagina'.) In a number of marsupials, males have a penis shaped like a two-pronged fork to match up with the two vaginas, and a separate sperm tube runs down each prong.
When an egg has been fertilized, it starts to move down the oviduct to the uterus.
In mammals other than marsupials (and egg laying 'Monotremes', like the platypus and spiny-anteater, or Echidna), the developing baby inside the mother is attached to her uterus by a cord leading to a placenta. This is a disc shaped patch where the baby's thinnest blood vessels lie close to those of the mother. It is where food substances in the mother's blood can pass into that of the baby. Also at the placenta, waste substances in the baby's blood pass into the mother's blood to be taken to her kidneys and lungs for removal.
The baby marsupial never stays still in the oviduct or uterus long enough to grow a cord and a placenta. Instead, glands in the walls of the uterus secrete a nutritious fluid, which bathes the outside of the tiny baby and provides its food. This would not be very efficient for large babies, and it is the reason why baby marsupials are born so small and such a short time after the egg has been fertilized. In some marsupials this 'gestation period' is only 12 days. Baby koalas weigh less than 0.5 gram when they are born. Baby kangaroos spend 3 weeks in the uterus, and are smaller than the size of your little finger when they are born.